/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */
package java.util.function;

import java.util.Objects;

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object)}.
 *
 * @param <T> the type of the input to the function
 * @param <R> the type of the result of the function
 * @since 1.8
 */
@FunctionalInterface
public interface Function<T, R> {

  /**
   * Applies this function to the given argument.
   *
   * @param t the function argument
   * @return the function result
   */
  R apply(T t);

  /**
   * Returns a composed function that first applies the {@code before}
   * function to its input, and then applies this function to the result.
   * If evaluation of either function throws an exception, it is relayed to
   * the caller of the composed function.
   *
   * @param <V> the type of input to the {@code before} function, and to the composed function
   * @param before the function to apply before this function is applied
   * @return a composed function that first applies the {@code before} function and then applies
   * this function
   * @throws NullPointerException if before is null
   * @see #andThen(Function)
   */
  default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
    Objects.requireNonNull(before);
    return (V v) -> apply(before.apply(v));
  }

  /**
   * Returns a composed function that first applies this function to
   * its input, and then applies the {@code after} function to the result.
   * If evaluation of either function throws an exception, it is relayed to
   * the caller of the composed function.
   *
   * @param <V> the type of output of the {@code after} function, and of the composed function
   * @param after the function to apply after this function is applied
   * @return a composed function that first applies this function and then applies the {@code after}
   * function
   * @throws NullPointerException if after is null
   * @see #compose(Function)
   */
  default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
    Objects.requireNonNull(after);
    return (T t) -> after.apply(apply(t));
  }

  /**
   * Returns a function that always returns its input argument.
   *
   * @param <T> the type of the input and output objects to the function
   * @return a function that always returns its input argument
   */
  static <T> Function<T, T> identity() {
    return t -> t;
  }
}
